Threaded washer

ABSTRACT

A threaded washer with a central bore for use with a bone screw is disclosed. The washer is threaded into a counterbore extending below the bone surface and into cancellous bone material. The external washer threads are tapered to provide improved purchase. The bone screw is inserted through the central bore of the washer and threaded into the fragment beyond the fracture. The washer provides improved compressive forces while permitting the bone screw head to be located beneath the bone surface.

This is a divisional of copending application Ser. No. 08/591,229, filedJan. 18, 1996.

FIELD OF THE INVENTION

The invention relates generally to the use of small screws in thereduction and compression of fractures and non-unions where the bones orbone fragments are small. These screws must provide sufficient stabilityand compression along the fracture line to promote healing. Forintra-articular applications, the screw head must not protrude above thebone surface in order to avoid interference with joint movement and withsurrounding tissue. The invention specifically relates to the use of awasher with external tapered threads to position the head of the smallscrew securely in the layer of cancellous material below the surface ofthe bone.

BACKGROUND OF THE INVENTION

In orthopedics, successful treatment of nonunion and fracture of smallbone fragments often involves the use of small screws. These screws areinstalled through both fragments in a direction substantially normal tothe fracture plane. The small screw must compress the fragmentssufficiently to promote healing.

Fractures resulting in small bone fragments often occur in bonesadjoining intra-articular regions, such as the scaphoid, other carpalbones, ends of long bones, and bones of the spine. It is important inthese applications that the head of the bone screw does not protrudefrom the outer surface of the bone, where the screw head could interferewith joint movement or damage surrounding tissue during articulation.

Standard small bone screws having threads only on the leading portion ofthe shank have been tried in this application. These screws are threadedinto pre-drilled holes until the head contacts the outer fragment. Thescrew is then turned until a force is generated compressing thefragments between the screw head and the threads, which engage the innerfragment. In intra-articular applications, the pre-drilled hole must becountersunk to prevent the head of the screw from protruding from thebone surface. The depth of this counterbore often exceeds the thicknessof the relatively thin bone cortex and penetrates to the softercancellous bone below. When tightened, the small screw head can sinkfurther into the cancellous bone, providing insufficient compressiveforces for proper union of the fragments.

Another approach to this problem is disclosed in U.S. Pat. No. 4,175,555to Herbert, which shows a small, headless screw utilizing threads ofdifferent pitch at the leading and trailing ends. The differential inpitch draws the bone fragments together as the screw is tightened.Because the screw head is replaced by the threads on the trailing end,the screw does not protrude from the bone and does not requirecountersinking. The threads on the trailing portion of the screw providesome purchase in the cancellous bone material. These screws, however,provide insufficient compression in many applications. Furthermore, asurgeon installing the screw must exercise care in starting each set ofthreads into the bone so that proper fracture reduction and compressionoccur simultaneously with the screw head reaching the proper depth.Otherwise, optimum fracture compression may occur too early, with thetrailing end protruding from the bone, or too late, with the trailingscrew threads engaging part of the inner fragment.

U.S. Pat. No. Re. 33,348 to Lower discloses a hip screw with anunthreaded shaft section and a leading threaded portion. A threadedsleeve member is retained on and slidable along the unthreaded shaftportion. The threads of the sleeve are designed to engage the relativelythick bone cortex of the femur. The threads on the leading portion andon the sleeve have different pitches. The assembled screw is installedas a unit, with compressive forces being generated as a result of thepitch differential as in Herbert '555. The two-piece design permits theunthreaded shaft section of the screw to back out through the sleevemember when bone absorption occurs, solving the problem of prior hipscrews being forced in the opposite direction, into the hip jointcapsule.

A method frequently used for the installation of small bone screws is tofirst install a guide wire in the bone in the location and orientationdecided upon for the screw. The wire passes through the surrounding softtissue, providing a guide for tools, such as screwdrivers, reamers anddrills, and a guide for the orthopedic implants, such as screws andwashers. Tools and implants used in such a procedure are cannulated,i.e. provided with a central bore through the long axis for placementover the wire.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a threaded washerfor countersinking a small bone screw below the surface of an outer bonefragment without compromising the compressive forces that the bone screwcan exert between the inner and outer fragments.

It is a further object to provide a threaded washer that providessufficient purchase in cancellous bone to support the head of a smallbone screw.

It is a further object of the invention to provide a threaded washer tosupport a small bone screw, wherein the washer thread is tapered toincrease the purchase in cancellous bone and to prevent the washer fromadvancing while inserting the screw.

It is a further object of the invention to provide a method forimmobilizing and compressing bone fragments using a threaded washer thatis countersunk into the cancellous material of the outer fragment tosupport the head of a small bone screw beneath the surface of thefragment in an intra-articular region.

In accordance with the invention, the noted drawbacks of existing meansof installing small bone screws are overcome by use of a threaded washerhaving an external, tapered thread with a thread form designed forpurchase in cancellous material, a central bore for insertion of thebone screw, and provision for receiving a driving means such as ahexagonal drive.

In one aspect of the invention, the threaded washer is used with acannulated screw. A guide wire is first inserted into the bone at thelocation selected for the screw. A cannulated reamer is then used toform a hole sufficiently deep to accommodate the threaded washer belowthe surface of the bone. The threaded washer is then inserted using acannulated driver, and driven into the cancellous bone material untilthe tapered thread is fully engaged in the hole. The cannulated screw isthen installed through the central bore of the threaded washer, anddriven using a cannulated driver until the head of the screw contactsthe threaded washer. Further turning of the cannulated screw has noeffect on the position of the tapered washer, but serves to reduce thefracture and apply compressive forces across the fracture plane.

The advantages of the threaded washer according to the invention residepartly in its universal applicability, since it can be used togetherwith standard cannulated screws.

The threaded washer according to the invention can be used inapplications where the bone screw must be inserted into the bone througha wall adjoining an intra-articular region, as is typically the case inscaphoid fractures. In such applications, it is important to countersinkthe head of the screw into the bone to avoid interference with jointarticulation. This must be done without compromising the compressiveforces applied across the fracture plane. With its improved purchase inthe cancellous material surrounding the counterbore, the threaded washerof the invention provides this added compressive strength while stillallowing the use of miniature screws with small screw heads.

Preferably the threaded washer and the bone screw have the same pitch.An advantage of the threaded washer over the use of existing screwshaving a pitch differential between the leading and trailing threads isthat fracture reduction is controlled independently of the depth of thescrew head beneath the bone surface. In the present invention, once thehead of the bone screw contacts the threaded washer, all furtherrotation of the screw reduces the fracture or applies compressive forcesacross the fracture plane, without changing the depth of the screw headbeneath the bone surface.

DESCRIPTION OF THE DRAWINGS

The invention will be disclosed more fully in connection with theaccompanying drawings in which:

FIG. 1 is a side elevation view in partial cross section of a threadedwasher according to the invention;

FIG. 2 is a top view of the threaded washer of FIG. 1;

FIG. 3 is a longitudinal section through a portion the threads of thethreaded washer;

FIG. 4 is a side elevation sectional view of a threaded washer accordingto the invention with a cannulated screw shown in the installedposition;

FIG. 5 is a cross-sectional view of a fractured bone after inserting aguide wire according to the invention;

FIG. 6 is a cross-sectional view of a fractured bone after reaming ahole for the threaded washer according to the invention;

FIG. 7 is a cross-sectional view of a fractured bone after inserting athreaded washer according to the invention; and

FIG. 8 is a cross-sectional view of a fractured bone after installingand tightening a cannulated screw according to the invention.

DETAILED DESCRIPTION

FIGS. 1 and 2 show a threaded washer 1 according to the invention. Asbest seen in FIG. 1, the threaded washer 1 comprises a hollowcylindrical body 2 with a trailing edge 3 and a leading edge 4. Acentral unthreaded bore 5 extends along a central axis 6 of thecylindrical body 2.

In the embodiment shown in FIGS. 1 & 2, a hexagonal socket 8 is providedin the central bore 5 at the trailing edge 3 for driving the threadedwasher with a hexagonal drive (not shown). Those skilled in the art willrecognize that other driving means, such as a double pin or a slot,could be used.

A tapered, external thread 9 is provided on the outer surface of thehollow cylindrical body 2. The thread form, including the tip 10 androot 11, is gradually reduced in size from the trailing edge 3 to theleading edge 4, to form a taper.

In one embodiment of the invention, the thread 9 is tapered 4.5 degreesper side. The taper provides increased purchase in cancellous bonematerial, and prevents the threaded washer from advancing while turninga bone screw that has been inserted through bore 5.

In the preferred embodiment, the tapered thread 9 of the washer hascutting flutes 14, shown in FIG. 1, to make the washer 1 self-tapping.The form of the thread 9 is a cancellous thread form; that is, a formdesigned for purchase in cancellous bone. FIG. 3 shows a cancellousthread form used in one embodiment of the invention. The thread leadingface 50, which transfers the compressive force between the threadedwasher and the surrounding cancellous bone tissue, has a small leadingface angle 51, for example 10 degrees. The trailing face 54 has atrailing face angle 55 greater than the leading face angle, for example30 degrees. This embodiment has a major diameter 20 of 5.5 mm and awhole tooth depth 58 of 0.625 mm. It will be understood, however, thatthe invention is in no way limited to this or any other threadconfiguration.

External chamfer 12, shown in FIG. 1, is provided on the tapered thread9 where it intersects the leading edge 4, in order to facilitateinsertion of the threaded washer, as described below. A second externalchamfer 13 is provided on the trailing edge of the washer to break thesharp corner.

In one embodiment, the threaded washer of the invention is fabricatedfrom a biologically inert metal such as titanium, a titanium alloy, oran implant quality stainless steel, for example ASTM-316L. A metallicwasher and screw are often removed in a subsequent surgical procedureafter the fracture has healed. For example, the washer and screw may beremoved to relieve discomfort caused by continued bone compressionacross the healed fracture plane.

In an alternative embodiment, the washer may be fabricated from abio-absorbable material. In this case, no subsequent implant removalprocedure is contemplated, and bone material is permitted to form aroundthe metallic screw as the washer is absorbed by the patient's system.Furthermore, compression on the bone fragments is gradually reduced asthe washer is absorbed.

Resorbable or degradable materials that may be used for fabricating thewasher include ceramics based, for example, on tricalcium phosphate,hydroxyapatite, calcium carbonate or combinations thereof.

Other materials which can be used for the washer according to theinvention include polymers such as highly purified polyhydroxyacids,polyamines, polyaminoacids, copolymers of amino acids and glutamic acid,polyorthoesters, polyanhydrides, amides, polydioxanone,polydioxanediones, polyesteramides, polymalic acid, polyesters of diolsand oxalie and/or succinic acids, polycaprolactone, copolyoxalates,polycarbonates or poly(glutamicco-leucine). Preferably usedpolyhydroxyacids comprise polycaprolactone polylactides in their variouschemical configuration [e.g. poly(L-lactide), poly(D-lactide),poly(L/D-lactide), poly(L/DL-lactide)], polyglycolide, copolymers oflactide and glycolide of various compositions, copolymers of saidlactides and/or glycolide with other polyesters, copolymers of glycolideand trimethylene carbonate, poly(glycolide-co-trimethylene carbonate),polyhydroxybutyrate, polyhydroxyvalerate, copolymers of hydroxybutyrateand hydroxyvalerate of various compositions.

Further materials to be used as additives are composite systemscontaining resorbable polymeric matrix and resorbable glasses andceramics based, for example, on tricalcium phosphate, hydroxyapatite,and/or calcium carbonate admixed to the polymer before processing.

FIG. 4 shows a threaded washer 1 of the invention with a cannulatedscrew 60. The screw shown in FIG. 4 is a 2.7 mm cannulated screw sold bySynthes (USA), the assignee of the present invention. Those skilled inthe art will recognize that: other-standard bone screws, both cannulatedand non-cannulated, can be used with the threaded washer of theinvention.

The bone screw 60 has a thread 61 designed for purchase in cancellousbone material. The bone screw thread 61 has a major diameter 62. Thecentral bore 5 of the threaded washer 1 is slightly larger than themajor diameter 62 of the bone screw threads, so that bone screw 60 canbe inserted into the threaded washer 1 without contacting the internalwall of the washer. In the embodiment shown in FIG. 4, the central bore5 of the washer 1 is approximately 0.3 mm larger than the major diameter62 of the bone screw thread 61.

Bone screw head 67 has a spherical undersurface 68. The sphericalundersurface tangentially engages the corners formed by the hexagonalsocket 8 and the top edge 3. Between the bone screw thread 61 and thebone screw head 67 is an unthreaded shaft portion 65. Self-drillingflutes 70 at the distal end of the bone screw thread 61 permit the screwto be inserted without pre-drilling or tapping.

The cannulated bone screw 60 shown in FIG. 4 furthermore has a centralbore 66 for receiving a guide wire 112 (shown in FIGS. 5-7), used duringthe implantation procedure. The bone screw head 67 has a cruciform slot69 for receiving a cruciform screw driver (not shown). The bone screw ispreferably fabricated from implant quality stainless steel.

FIGS. 5-8 illustrate a procedure for using the threaded washer of theinvention together with a cannulated bone screw, for reduction andfixation of a small bone fracture. Those skilled in the art willrecognize that other procedures for implanting the threaded washer,using cannulated or non-cannulated screws, are possible. It willfurthermore be understood that the use of the washer is not limited tothe use illustrated.

FIG. 5 shows the initial steps in preparing the bone for implantation ofthe threaded washer and cannulated screw.

Fracture plane 101 is shown separating outer fragment 105 from innerfragment 106. At the location selected for the implantation, thefracture plane 101 is within the cancellous region 102 of the bone,beneath the cortex 103.

A self-drilling guide wire 112 is inserted into the bone using a drill(not shown) in an orientation substantially perpendicular to thefracture plane 101. The guide wire has a fluted tip 110 to facilitatedrilling. The diameter of the guide wire 112 is chosen to be the properdiameter for the cannulated screw 60; for example, with a 3.0 mmcannulated screw, a 1.1 mm guide wire may be used. The guide wire 112passes through the cortex 103, through the outer fragment 105, andpenetrates the inner fragment 106 sufficiently to permit the thread 61of the cannulated screw 60 (FIG. 4) to completely engage the innerfragment. A drilling depth gauge (not shown) may be used to insure aproper guide wire depth.

As shown in FIG. 6, a tapered counterbore 120 is reamed in a positionconcentric to the guide wire 112 using a cannulated tapered reamer (notshown). The reamer is guided using the guide wire 112 in the mannerknown in the art. The depth of the counterbore may be controlled using agauging mark on the reamer (not shown) that aligns with the outersurface of the cortex 103.

The threaded washer 1 is installed in the counterbore 120 as shown inFIG. 7. The threaded washer 1 is first assembled over guide wire 112. Acannulated hexagonal driver (not shown) is then assembled over the guidewire 112 above the threaded washer 1 and engaged with the hexagonalsocket 8 of the threaded washer. Using the hexagonal driver, thethreaded washer is rotated in the counterbore 120, engaging the thread 9with the bone material. Cutting flutes 14 (FIG. 1) permit the taperedthread 9 to cut through cortex 103 and cancellous bone 102. The washer 1is rotated until thread 9 is fully engaged in tapered hole 120.

In the embodiment in which the washer 1 is fabricated of a bioabsorbablematerial, the use of a self-tapping thread with cutting flutes may notbe possible. In that case, the counterbore is tapped using a tapered,cannulated tap before implanting the threaded washer.

FIG. 8 shows the threaded washer and cannulated screw after installationis complete. The cannulated screw 60 is assembled over the guide wire(not shown), and a cannulated cruciform screwdriver (not shown) is thenassembled over the guide wire above the cannulated screw and inengagement with the cruciform slot 69 of the cannulated screw. Slidingon the guide wire 112, the cannulated screw is passed through thecentral bore 5 of threaded washer 1 until screw threads 61 engage thecancellous bone material 102 of the outer fragment 105 surrounding theguide wire. The cruciform screwdriver is then used to rotate thecannulated screw 60 until threads 61 pass through the outer fragment 105and are entirely engaged in inner fragment 106. The unthreaded shaftportion 65 of the cannulated screw passes through the washer 1 withoutcontact. The shaft portion 65 fully contacts the bone materialimmediately above and below the fracture plane 101, preventing lateralmovement between the inner and outer fragments.

After the cannulated screw 60 is driven into the bone, the undersurface68 of the screw head 67 contacts the threaded washer 1 at the cornersformed by the hexagonal socket 8 and the top edge 3. The fixed depth ofthreaded washer 1 below the outer surface of the bone ensures that thecannulated screw head 67 will be countersunk to permit jointarticulation and prevent soft tissue injury. Further rotation of thecannulated screw 60 draws the inner fragment 106 upward toward thethreaded washer 1, reducing the fracture 101. The cannulated screw 60 isthen rotated until the proper compressive force is applied across thefracture plane 101.

The compressive force can be increased or decreased without affectingthe axial position of the cannulated screw 60 in the outer fragment 105of the bone. Because the washer thread 9 is tapered, the washer 1 isrestrained from advancing due to friction while the bone screw 60 istightened against the threaded washer 1.

What is claimed is:
 1. A method for internal fixation of a first bonefragment to a second bone fragment, comprising:reaming the first bonefragment to produce a counterbore; installing a washer having a centralhole and an external tapered thread into said counterbore; inserting abond screw having a head through said central hole in said washer;rotating said bond screw so that a bone screw thread passes through thefirst bone fragment and engages the second bond fragment, and the bonescrew head engages said washer; whereby said bone screw exerts acompressive force between the first and second bone fragments.
 2. Amethod of fixation of an outer bone fragment to an inner bone fragment,comprising:inserting a guide wire through the outer fragment and intothe inner fragment; reaming a counterbore in the outer fragment;installing a washer having a central hole and an external, taperedthread into said counterbore; installing a cannulated, self-drillingbone screw through said central hole of said washer until a thread ofsaid bone screw engages the second bone fragment and a head of said bonescrew contacts said washer.
 3. The method of claim 2, further comprisingthe step of turning said bone screw after said head of said bone screwcontacts said washer, thereby reducing a gap between said bone fragmentsand compressing said bone fragments together.
 4. The method of claim 2,wherein said washer is bioabsorbable.
 5. The method of claim 4, furthercomprising the step of tapping said counterbore to accept said washer.6. The method of claim 1, further comprising the steps of turning saidbone screw after said head of said bone screw contacts said washer,wherein said head of said bone screw resides at a depth inside saidouter fragment, thereby reducing a gap between said bone fragments andcompressing said bone fragments together.
 7. The method of claim 6,wherein the step of turning said bone screw after said head of said bonescrew contacts said washer further comprises said head of said bonescrew maintaining said depth inside outer bone fragment.